Radiometric measuring arrangements are applied in industrial measurements technology, for example, for measuring a fill level of a fill substance in a container, for monitoring exceeding or subceeding of a predetermined fill level of a fill substance in a container, or for measuring a density of a fill substance.
Radiometric measuring arrangements are usually applied when conventional measuring methods are not applicable due to especially rough conditions at the measuring location. Very frequently present at the measuring location are extremely high temperatures and pressures or chemically and/or mechanically very aggressive environmental influences, which make use of other measuring methods impossible.
A fundamental principle of radioactive measurements technology is that one or more radioactive radiators, such as e.g. Co60 or Cs137 preparations, are positioned at a measuring location in such a manner that radiation emitted by them penetrates a region to be registered metrologically, e.g. a part of a container filled with a fill substance, and a radiation intensity emerging on a container side lying opposite the radiator is measured with an appropriate detector, e.g. a scintillation detector. The emerging radiation intensity depends on the geometric arrangement and the absorption along the path traveled by the radiator to the detector. The latter is in the case of fill level measurement dependent on the fill level of the fill substance located in the measuring path in the container and in the case of density measurement on the density of the fill substance located in the measuring path. As a result, the emerging radiation intensity is a measure for the current fill level, respectively the current density, of the fill substance in the container.
There are a large number of industrial applications, in the case of which, with time, accretion can form on the inner walls of the container. The terminology, accretion, refers here to deposits of material attached to the inner walls. Accretion arises, for example, when the fill substance located in the container, or, in the case of tubular containers, flowing through the container, clings to the inner wall, and thereby establishes an accretion layer on the inner wall of the container. With time, other deposits or attached material causes the layer to become increasingly thicker.
In such case, there is the problem that the measuring path through the container leads through the layer of accreted material both in the case of its entry into the container as well as also in the case of its exit from the container. Correspondingly, both entrance side as well as also exit side, a part of the radiation sent through the container is absorbed in the accretion layer. Since the accretion layer has a higher density deviating, as a rule, from the medium, it absorbs radioactive radiation more strongly than the fill substance. In this way, the radiation intensity emerging in measurement operation is increasingly reduced, and, thus, the measurement result becomes more and more corrupted. Due to the accretion related, lower measured radiation intensity, a too high fill level, respectively a too high density, is measured. If this measurement error is not recognized, such can have fatal effects, especially in safety-relevant applications.
There is, consequently, a need to detect accretion formation in containers as early as possible. Exactly in applications in which radiometric measuring systems are applied, it is, as a rule, not, or not directly, possible to open the container, in order, in given cases, to detect accretion formation early.
Moreover, container cleaning, in the case of which accretion is removed, is, as a rule, especially complicated and expensive, particularly in these applications.
Described in U.S. Pat. No. 3,594,575 A1 is a fill level measuring arrangement, in the case of which, in given cases, accretion formation arisen in the container can be taken into consideration by a new calibration of the measuring arrangement executed on-site. In the new calibration, the dependence of the measured radiation intensities on the fill level is determined anew in the case of a currently present container state. In an ongoing operation, it is, however, not possible to detect whether accretion has formed. Insofar, as there remains even here the possibility of an unrecognized measuring result corruption due to accretion formation.